Sm(Co, Fe, Cu, Zr, C) compositions and methods of producing same

ABSTRACT

Carbon addition to the rapidly solidified, preferably melt spun, alloy system of Sm(Co, Fe, Cu, Zr) provides for good isotropic magnetic properties. Importantly, these alloys are nanocomposite in nature and comprise the SmCoC 2  phase. Thermal processing of these materials can achieve good magnetic properties at lower temperatures and/or shorter processing times than conventional Sm(Co, Fe, Cu, Zr) powders for bonded magnet application.

This application claims the benefit of Provisional application Ser. No.60/106,360, filed Oct. 30, 1998.

FIELD OF THE INVENTION

The present invention relates to magnetic materials, and moreparticularly relates to magnetic nanocomposite materials includingsamarium, cobalt, iron, copper, zirconium and carbon which havefavorable magnetic properties and are suitable for making bondedmagnets.

BACKGROUND INFORMATION

The Sm(Co,Fe,Cu,Zr)_(z) sintered magnets exhibit outstanding thermalstability and high energy products at elevated temperatures due to theirhigh Curie temperature and spontaneous magnetization. See K. J. Strnat,Proceeding of IEEE, Vol. 78 No. 6 (1990) pp. 923; and A. E. Ray and S.Liu, Journal of Materials Engineering and Performance, Vol. 2 (1992) pp.183. However, sintered magnets are very hard and brittle, which makesfinal finishing very costly and may reduce the production yield ratesignificantly. The near net-shape production enables Sm(Co,Fe,Cu,Zr)_(z)bonded magnets to be used for many sophisticated applications. In ourprevious work, we focused on the magnetic properties and developedSm(Co,Fe,Cu,Zr)_(z) powders for bonded magnet applications usingconventionally cast alloys. See W. Gong, B. M. Ma and C. O. Bounds, J.Appl. Phys. Vol. 81 (1997) pp. 5640; W. Gong, B. M. Ma and C. O. Bounds,J. Appl. Phys. Vol. 83 (1998) pp. 6709; and W. Gong, B. M. Ma and C. O.Bounds, J. Appl. Phys. Vol. 83 (1998) pp. 6712. Our studies ranged fromthe effects of phase transformation, solid solution and agingheat-treatments, the particle size and distribution, and theconsolidating pressure on the magnetic properties of bonded magnets.

Carbon is a common impurity found in the conventional castSm(Co,Fe,Cu,Zr)_(z) alloys. It forms carbides and exhibits a negativeimpact on the intrinsic coercivity, H_(ci), and maximum energy product,(BH)_(max). Recently, C additions have been found to change the latticeparameters and, consequently, the magnetic anisotropy of manySm₂Fe₁₇-based compounds prepared by casting. See B. G. Shen, L. S. Kong,F. W. Fang and L. Cao, J. Appl. Phys. Vol. 75 (1994) pp. 6253. Moreover,the melt spinning technique has been applied to this alloy system andhas shown many interesting results. See Z. Chen and G. C. Hadjipanayis,J. Magn. Magn. Mate. Vol. 171 (1997) pp. 261. It is of interest toincorporate carbon into the conventional Sm(Co,Fe,Cu,Zr)_(z) alloyssystem and to compare its impact on the structural and magneticproperties of materials prepared by different synthesizing methods.

It is the object of the present invention to provide compositionsnanocomposite in nature.

It is the further object of the present invention to obtain isotropicmagnetic properties.

It is an object of the present invention to obtain compositionscomprising, preferably predominately, the SmCoC₂ phase.

Another object of the present invention is to provide compositions whichrequire short thermal processing time and or low processing temperatureto fully develop favorable magnetic properties.

These and other objects of the present invention will become moreapparent from the following description and examples.

SUMMARY OF THE INVENTION

The magnetic nanocomposite compositions of the present invention includesamarium (Sm) and cobalt (Co), copper (Cu) and iron (Fe), zirconium (Zr)and carbon (C). Preferably, compositions having a predominately SmCoC₂phase. These compositions provide powder-bonded type magnets withfavorable magnetic properties. The compositions are preferably rapidlysolidified by conventional methods, most preferably by melt spinning,followed by thermally treating the material to form crystalline magneticphases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a series of X-ray powder diffraction patterns ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0), where x=0 to0.15, as-spun ribbons. Diffraction marked with (•) are the TbCu₇structure.

FIG. 2 is a series of X-ray powder diffraction patterns ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) ribbons where x=0or 0.05 after various heat treatments. Diffraction peaks marked with(•), (+) and (*) are the TH₂Zn₁₇, SmCoC₂ and ZrC structure,respectively.

FIG. 3 is a series of DTA scans onSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) samples showingthe endothermic (•)and exothermic(+) peaks of the SmCoC₂ phase.

FIG. 4 is a plot of coercivity, namely the variation of the H_(ci) ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) ribbons as afunction of the carbon content, x, after a heat-treatment temperatureranged from 700 to 800° C. for 5 minutes.

FIG. 5 is a series of magnetization curves and magnetic properties ofSm(Co_(0.62)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(0.05))_(8.0) heat treatedribbons.

DETAILED DESCRIPTION OF THE INVENTION

Compositions of the present invention are of the formula:

Sm(Co_(1-u-v-w-x)Fe_(u)Cu_(v)Zr_(w)C_(x))_(z):

wherein x, u, v, w, and (1-u-v-w) are generally in the range shown byTABLE A.

TABLE A C Fe Cu Zr Co x u v w l-u-v-w z Broadest Range 0.001-0.250.01-0.4  0.01-0.20 0.001-0.20  balance 6.0-9.0 Preferable 0.005-0.200.10-0.35 0.03-0.08 0.01-0.04 balance 6.5-8.5 Most Preferable  0.01-0.120.2-0.3 0.05-0.07 0.02-0.03 balance 7.0-8.5

Zirconium may also be utilized in combination with titanium, hafnium,tantalum, niobium, and vanadium. Further, these elements, alone or incombination, may be substituted for Zirconium.

The magnetic materials of the present invention are preferably producedby a rapid solidification and thermal treatment process. Rapidsolidification is achieved by quickly cooling the compositions from themolten state by known techniques such as melt spinning, jet casting,melt extraction, atomization and splat cooling. Preferred for use hereinis melt spinning. After rapid solidification, the material is thermallytreated.

Processing temperatures and duration ranges for thermal treatment arefrom about 400 to about 1200° C. for 0 to about 24 hours, preferablyfrom about 500 to about 1150° C. for from about 1 minute to about 1hour, and most preferably from about 700 to about 800° C. for from about1 minute to about 10 minutes.

For bonded magnets prepared with the compositions of the presentinvention, operational ranges are generally from about 70 to about 500°C., preferably from about 40 to about 400° C., and most preferably fromabout 25 to about 300° C. Conventional methods for preparing bondedmagnets can be utilized and generally comprise the steps of providing acomposition of the present invention in powder form, mixing the powderwith a binder and curing.

The following examples illustrate various aspects of the presentinvention and are not intended to limit the scope thereof.

Experimental

In this work, Applicants report the effects of carbon-addition on themagnetic and structural properties ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0), where x=0 to0.15. Emphasis is foucused on the comparison of the characteristics ofmaterials prepared by the conventional casting and melt spinning.

The effects of C additions on the phase transformation and magneticproperties of Sm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0),where x ranged from 0 to 0.15, melt spun ribbons and and cast alloyshave been studied by x-ray diffraction (XRD), differential thermalanalysis (DTA), and vibrating sample magnetometer (VSM). In addition tothe Th₂Zn₁₇ structure, two additional compounds, namely, the ZrC andSmCoC₂, were detected by XRD after a thermal treatment over about 700 toabout 1160° C. The DTA scans indicated exothermic and endothermic peaksof the SmCoC₂ phase occur at about 740 and 950° C., respectively. Theamount of SmCoC₂ is found to increase with increasing nominal C contentand plays a critical role to the formation of amorphous precursoralloys. The as-spun ribbons were highly crystalline at x=0 and becamemostly amorphous at x=0.10. An intrinsic coercivity, H_(ci), of 3.0 kOewas obtained for the as-spun ribbons with x=0.05. After an optimumheat-treatment, the H_(ci) of the ribbons with x=0.01 was increased to 8kOe. Cast alloys of identical chemical compositions were also solutiontreated and precipitation hardened. At x=0 for the cast alloy, a B_(r)of 10.8 kG, H_(ci) of 24 kOe, H_(c) of 9.8 kOe and (BH)_(max) of 27MGOe, were obtained after an optimum heat-treatment. Unlike melt spunmaterials, the hard magnetic properties of the conventionally castalloys were found to decrease with increasing C-content and governed bya different magnetization reversal mechanism.

The Sm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) master alloyswere prepared by both the conventional vacuum induction melting andarc-melting. The melt-spun ribbons were made of master alloys bymelt-spinning using a quartz tube with an orifice diameter of about 0.7mm and a wheel speed in excess of 45 m/s. These ribbons were then sealedin a quartz tube under vacuum of 10⁻⁵ Torr and isothermally treated attemperatures ranging from about 700 up to 800° C. for 5 minutes. Themaster alloys were also solution treated at temperatures of about1100-1200° C. for 12 hours, precipitated hardened at temperatures ofabout 800 to 900° C. for 8 hours, then slowly cooled at a rate of about1° C./min to about 400° C. for 4 hours. A Perkin Elmer DifferentialThermal Analyzer (DTA) was used to determine the phase transformationtemperatures of samples. The crystal structure of the ribbons and masteralloys were determined by a Siemens x-ray diffractometer, with a Co Kαradiation, in conjunction with a Hi-Star Area Detector. Magneticproperties of the ribbons and powdered alloys (−200 Mesh) were measuredby a Vibrating Sample Magnetometer (VSM). For anisotropic powders,cylindrically shaped magnets were prepared by mixing powders withparaffin, aligned in a dc magnetic field with a maximum field of 30 kOe,melt then solidified. Magnets were pulse magnetized with a peak field of100 kOe prior to any measurements. A theoretical specific density, ρ, of8.4 g/cm³ and demagnetization factors were used for calculating 4πM,B_(r) and (BH)_(max), wherein M represents magnetization, B_(r)represents magnetic remanence, and (BH)_(max) represents maximum energyproduct.

Results and Discussion

Shown in FIG. 1 are the XRD patterns of the as-spunSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) where x rangesfrom 0 to 0.15, ribbons as a function of the carbon content. At x=0, theribbons were completely crystalline. These diffraction peaks can beindexed to the characteristic peaks of the hexagonal TbCu₇ mixed with asmall amount of α-Fe. This result is similar to the structure change ofmelt spun Sm₂(Co_(1-x)Mn_(x))₁₇ from the Th₂Zn₁₇ structure to the TbCu₇when prepared above a critical wheel speed. See H. Saito, M. Takahashiand T. Wakiyama, J. Magn. Magn. Mate. Vol. 82 (1989) pp. 322. It wasfound that the characteristic peaks of TbCu₇ phase gradually diminishand become fully amorphous when the carbon content was increased from 0to 0.15. This suggests that the C addition, when above a critical level,suppresses the formation of TbCu₇ and α-Fe.

Shown in FIG. 2 are the XRD patterns ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) ribbons in theas-spun and after various thermal treatments. Crystalline phase with adisordered TbCu₇ phase and α-Fe were observed when treated attemperatures from about 700 to 800° C. for 5 minutes. The TbCu₇ phasetransformed to a rhombohedral Th₂Zn₁₇, when the samples were heated toabout 1160° C. for 16 hours. When compared to the XDR characteristicpeaks of Sm(Co_(0.67)Fe_(0.25)Cu_(0.06)Zr_(0.02))_(8.0), i.e. at x=0,heat treated at the same temperature, two additional phases, namely ofSmCoC₂ and ZrC, were also detected in the ribbons with a nominalcompositional of Sm(Co_(0.62)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(0.05))_(8.0),i.e. x=0.05.

Depending on the rare earth component, the RCoC₂, where R is the rareearth, forms two different crystallographic structures. It forms amonoclinic structure with light rare earths and orthorhombic structurewith heavy rare earths. See W. Schafer, W. Kockelmann, G. Will, P. A.Kotsanidis, J. K. Yakinthos and J. Linhart, J. Magn. Magn. Mate. Vol.132 (1994) pp. 243; and O. I. Bodak, E. P. Marusin and V. A. Bruskov,Sov. Phys. Crystallogr. 25 (1980) pp. 355. The SmCoC₂ phase also formsreadily in the SmCo₅ magnets if the raw materials contain more than 0.03wt % carbon or if magnets were contaminated by the carbon containingprotection fluid during milling of the powder. See M. F. De Campos andF. J. G. Landgraf, Proc. 14th Inter. Work. Rare Earth Magnets and Appl.,Vol. 1 (1996) pp. 432. The RCoC₂ is the only ternary phase detected inthe Sm—Co—C isoplethic section at about 900° C. See H. H. Stadelmaierand N. C. Liu, Z. Metallkde. 76 (1985) pp. 585. The DTA scan of theSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) alloys, shown inFIG. 3, reveals an endothermic peak during heating and an exothermicpeak during cooling at about 950 and 740° C., respectively. Thedifferential temperature, ΔT, of the SmCoC₂ peaks inSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) alloys increaseswith increasing x. Alloys with a higher carbon content seem to formSmCoC₂ more readily. A higher amount of SmCoC₂ may be related to theease of formation of amorphous precursor alloys.

The Sm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) ribbons wereheat-treated at about 700, 720, 760 and 800 for 5 minutes. Shown in FIG.4 are the variation of H_(ci) with the carbon content, x, at variousthermal processing temperatures. At x=0, H_(ci) values of 2.0 to 3.5 kOewere obtained after various thermal processing. Without carbon addition,the H_(ci) appears to be insensitive to the thermal processingtemperature due to the crystalline nature of the precursor alloy. Atx=0.01, the H_(ci) increases from 2 kOe in the as-spun state to 5.6 kOeat 700° C., peaks to approximately 8 kOe at 720° C., then decreases to7.0 and 6.5 kOe when thermally processed at 760 and 800° C. Similartrends can be observed for x up to 0.05. At x=0.05, an H_(ci) of 3.0 kOewas obtained on the as-spun ribbons and a H_(ci) of 6.5 kOe was obtainedafter 760° C. treatment. Similarly, at x=0.10, an H_(ci) of nearly 0 kOewas obtained in the as-spun state and agrees reasonably well with theamorphous nature of the as-spun materials. An H_(ci) of 6.5 kOe wasobtained after being thermally processed at 800° C. At high carboncontent, namely x=0.15, limited H_(ci) can be developed within thetemperature range studied regardless of the amorphous nature of theprecursor alloy ribbons. Based on these results, it suggests that thedesired carbon content ranges from x=0.005 to 0.1 and the optimumthermal processing temperature seems to lie between about 720 to 760° C.This optimum processing temperature coincides considerably well with theexothermic peak of SmCoC₂ observed at about 740° C. as previously shownin FIG. 3. The carbon content and the thermal processing temperature aretwo important factors requiring control to develop the nanocomposite orthe desired microstructure for the hard magnetic properties of thecomposition studied.

Shown in FIG. 5 are the magnetization curves, measured isotropically, ofthe Sm(Co_(0.62)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(0.05))_(8.0) ribbons inthe as-spun, and after thermal process 700 and 760° C. A B_(r) of 6.2kG, H_(ci) of 3.0 kOe, H_(c) of 1.7 kOe and (BH)_(max) of 3.0 MGOe wereobtained on the as-spun ribbons. A B_(r) of 7.6 kG, H_(ci) of 3.8 kOe,H_(c) of 3.0 kOe and (BH)_(max) of 6.0 MGOe were obtained after theribbons were heat-treated at 700° C. A B_(r) of 7.5 kG, H_(ci) of 6.9kOe, H_(c) of 3.9 kOe and (BH)_(max) of 7.2 MGOe were obtained afterbeing processed at 760° C. A (BH)_(max) of 7.2 MGOe, in conjunction withthe high T_(c), makes these materials attractive for the bonded magnetapplications and deserve further investigation.

No permanent magnetic properties could be developed until a combinedsolid solution treatment at about 1160° C. and precipitation hardeningat about 850° C. were adopted. It appears that the hard magneticproperties of Sm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0)follow the traditional mechanism: a cellular microstructure with finelyprecipitated platelets as pinning centers for magnetization reversal.Listed in Table I are the B_(r), H_(ci), H_(c), and (BH)_(max), measuredanisotropically, of fully processedSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0). Unlike the meltspun materials, the B_(r), H_(ci),H_(c) and (BH)_(max) ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) diminishdrastically with the increasing carbon content. It is hypothesized thatalloy with high carbon content may form undesired phases and hinder theformation of cellular structure and the desired precipitated phases aspinning centers for the magnetization reversal.

Table I shows Magnetic properties ofSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) powdered masteralloys after a solid solution treatment and precipitation hardening

TABLE I x B_(r) H_(ci) H_(cb) (BH)_(max) (at %) (kG) (kOe) (kOe) (MGOe)0 10.8 24 9.8 27 0.005 10.7 16 8.7 26 0.05 10.2 3.2 3.0 9 0.10 2.0 0.50.2 ˜0 0.15 2.0 0.5 0.1 ˜0

Conclusions

The effects of C additions on the phase transformation and magneticproperties of Sm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0),where x ranged from 0 to 0.15, melt spun ribbons and cast alloys havebeen studied. At low carbon concentration, the as-spunSm(Co_(0.67-x)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(x))_(8.0) consists of theTbCu₇ structure with a minor amount of (α-Fe. In additional to theTh₂Zn₁₇ structure, two additional compounds, namely, the ZrC and SmCoC₂,were detected in the melt spun by XRD after a thermal treatment over 700to 1160° C. The amount of SmCoC₂ is found to increase with increasingnominal C-content and plays a critical role in the formation of theamorphous precursor alloy. Thermally processed ribbons were found toexhibit isotropic magnetic properties. A B_(r) of 7.5 kG, H_(ci) of 6.9kOe, H_(c) of 3.9 kOe and (BH)_(max) of 7.2 MGOe were obtained on anoptimally processedSm(Co_(0.62)Fe_(0.25)Cu_(0.06)Zr_(0.02)C_(0.05))_(8.0). Unlike melt spunmaterials, the hard magnetic properties of the conventionally castalloys were found to decrease with increasing C-content.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

What is claimed:
 1. A nanocomposite magnetic material of the formula:Sm(Co_(1-u-v-w-x) Fe_(u)Cu_(v)Zr_(w)C_(x))_(z), where x is from about0.001 to about 0.25, u is from about 0.01 to about 0.4, v is from about0.01 to about 0.20, w is from about 0.001 to about 0.20, and z is fromabout 6.0 to about 9.0, wherein the material comprises the SmCoC₂ phase.2. The nanocomposite magnetic material of claim 1, wherein x is fromabout 0.005 to about 0.20, u is from about 0.10 to about 0.35, v is fromabout 0.03 to about 0.08, w is from about 0.01 to about 0.04, and z isfrom about 6.5 to about 8.5.
 3. The nanocomposite magnetic material ofclaim 1, wherein x is from about 0.01 to about 0.12, u is from about 0.2to about 0.3, v is from about 0.05 to about 0.07, w is from about 0.02to about 0.03, and z is from about 7.0 to about 8.5.
 4. Thenanocomposite magnetic material of claim 1, wherein the material is inpowder form.
 5. The nanocomposite magnetic material of claim 4, whereinthe powder has been produced by rapid solidification and thermaltreatment.
 6. The nanocomposite magnetic material of claim 5, whereinthe powder is magnetically isotropic.
 7. A method of making ananocomposite magnetic material comprising: a) providing a moltencomposition comprising: Sm(Co_(1-u-v-w-x)Fe_(u)Cu_(v)Zr_(w)C_(x))_(z)where x is from about 0.001 to about 0.25, u is from about 0.01 to about0.4, v is from about 0.01 to about 0.20, w is from about 0.001 to about0.20, and z is from about 6.0 to about 9.0; b) rapidly solidifying themolten composition to form a product at least comprising a partiallyamorphous phase; and c) thermally treating the product at a temperatureranging from about 400° C. to about 1200° C. for from about 1 minute toabout 24 hours.
 8. The method of claim 7, wherein the temperature rangesfrom about 500° C. to about 1150° C. for from about 1 minute to about 1hour.
 9. The method of claim 8, wherein the temperature ranges fromabout 700° C. to about 800° C. for from about 1 minute to about 10minutes.
 10. A bonded magnet comprising the nanocomposite material ofclaim
 1. 11. A method of making a bonded magnet comprising: a) providingthe nanocomposite magnetic material of claim 1 in powdered form; b)mixing the powdered nanocomposite magnetic material with a binder; andc) curing the binder to form the bonded magnet.
 12. The nanocompositemagnetic material of claim 1, wherein x is from about 0:005 to about0.10.
 13. A nanocomposite magnetic material made according to the methodof claim
 7. 14. A nanocomposite magnetic material made according to themethod of claim
 8. 15. A nanocomposite magnetic material made accordingto the method of claim 9.